In recent years, alkaline batteries have been expected to have improved discharge performance in an entire range of load from light load to heavy load, and in particular, the improvement in discharge performance in an intermediate load has been demanded. The main application corresponding to discharge under a heavy load is digital still cameras, and the main application corresponding to discharge under an intermediate load is portable music devices. It is difficult, however, to improve the discharge performance in the intermediate load (i.e., in AA-size batteries, current of about 0.1 to 0.4 A), and there has been a demand for a technique for improving the intermediate load discharge performance without degrading the discharge performance in the other range of load.
Manganese dioxide included in a positive electrode for alkaline batteries contains structural water (or water of crystallization) in the crystal structure thereof. Conventionally, in view of improving heavy load discharge performance, the use of manganese dioxide containing an increased amount of structural water has been proposed.
For example, Patent Document 1 proposes the use of manganese dioxide having a weight loss rate when heated from 200 to 400° C. of 2.7% or more. In such manganese dioxide, the weight loss rate when heated from 150 to 400° C. is estimated to be 3.2% or more.
Non-patent Document 1 proposes that the synthesis conditions for manganese dioxide be controlled to change the crystal structure of the manganese dioxide, thereby to improve the heavy load discharge performance.
Non-patent Document 2 discloses that the weight loss rate from 150 to about 400° C. reflects the amount of Mn vacancies in the solid phase of manganese dioxide. When there is a large amount of Mn vacancies, the migration of hydrogen ions in the solid phase of manganese dioxide is facilitated. Accordingly, it is assumed that when a large amount of current flows, the polarization during discharge is reduced, resulting in a good discharge performance. On this assumption, the synthesis conditions under which the Mn vacancies are increased are examined.
Patent Document 2 proposes that in an alkaline zinc battery, in view of improving the characteristics when the battery is continuously discharged at 1 A, the pore volume in the manganese dioxide be 0.035 cm3/g to 0.050 cm3/g.
Patent Document 3 proposes a method for preparing manganese dioxide suited for heavy load discharge, by way of controlling the electrolytic synthesis conditions.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-186127
Patent Document 2: Japanese Laid-Open Patent Publication (Translation of PCT Application) No. 2005-520390
Patent Document 3: Japanese Laid-Open Patent Publication (Translation of PCT Application) No. 2002-533288
Non-patent Document 1: Collection of 44th Battery Symposium Lecture Summaries, pp. 656-657
Non-patent Document 2: Paul Reutschi et al., J. Electrochem. Soc., Vol. 135, No. 11 (1988), pp. 2663-2669
However, sufficient improvement of both heavy load discharge performance and intermediate discharge performance cannot be achieved by merely increasing the structural water in manganese dioxide as conventionally proposed. In order to achieve further improvement of both heavy load discharge performance and intermediate discharge performance of alkaline batteries, it is considered necessary to control not only the content of structural water but also the relationship between the content of structural water and other physical properties of manganese dioxide.